Updated at 4:43 p.m. ET: BEIRUT, Lebanon -- A huge car bomb explosion in Beirut on Friday killed a top Lebanese security official whose investigations implicated Syria and Hezbollah in the assassination of former Lebanese prime minister Rafik al-Hariri seven years ago.

The rush-hour bomb in the center of the Lebanese capital killed eight people and wounded about 80 others, heightening fears that Syria's war is spilling over into Lebanon.

Among the dead was Wissam al-Hassan, the head of a Lebanese intelligence agency who had also uncovered a recent bomb plot that led to the arrest of a pro-Syrian Lebanese politician, a Lebanese official said.

Al-Hassan was a close aide to Hariri, a Sunni Muslim who was killed in a 2005 bomb attack in downtown Beirut. Al-Hassan's investigation into Hariri's death uncovered evidence that implicated Syria and Hezbollah in the killing.

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It was also not clear if the explosion targeted any political figure in Lebanon's divided community but it occurred at a time of heightened tension between Lebanese factions on opposite sides of the Syria conflict.

Ambulances rushed to the scene in the Ashafriyeh district, a mostly Christian area, as smoke rose from the area. 

The explosion ripped through the street where the office of the anti-Damascus Christian Phalange Party is located near Sassine Square.

Reuters

Phalange leader Sami al-Gemayel, a staunch opponent of Syrian President Bashar al-Assad and a member of parliament, condemned the attack.

"Let the state protect the citizens. We will not accept any procrastination in this matter, we cannot continue like that. We have been warning for a year. Enough," said Gemayel, whose brother was assassinated in November 2006.

Several cars were set on fire by the explosion and the front of a multi-story building was badly damaged. Residents ran about in panic looking for relatives while others helped carry the wounded to ambulances, Reuters reported. 

Slideshow: Bombing in Beirut

Reuters

Huge blast explodes in a central Beirut street injures dozens, kills at least eight.

Launch slideshow

Pope tells Christians in Beirut: 'Be peacemakers'

Security forces blanketed the area.

Witness Danny Rizkallah told NBC News the blast took place close to the headquarters of a Lebanese opposition political party with links to Syria rebels and close to the scene of the 1982 assassination of then president-elect Bachir Gemayel. The affluent, largely Christian, district is also home to the American University of Science and Technology (AUST).

He said he was having lunch nearby when the blast lifted him from his chair. “It was an incredibly powerful explosion,” he said. “I knew immediately it was a bomb because it has such a different sound to shelling.”

“I rushed around the corner to see what happened there were lots of people injured by broken glass from the windows of nearby stores. It did a great deal of damage to nearby buildings and there was a lot of glass.

Hasan Shaaban / Reuters

Burning cars and damages are seen at the site of an explosion in Ashafriyeh, central Beirut, October 19, 2012.

“For this to happen is shocking because we really thought this sort of thing had stopped in Beirut, and for it to happen in the Christian district is also very unusual. I really don’t know who is behind this, or why. Our politics is very messed-up.”

The last bombing in Beirut was in 2008 when three people were killed in an explosion that damaged a U.S. diplomatic car. 

U.S. officials are condemning the attack "in the strongest terms," calling the blast a terrorist attack.

"We condemn this act of terrorism," State Department spokeswoman Victoria Nuland said.

"There is no justification for such violence," she added. "We obviously express our heartfelt sympathies for the families and the loved ones of those who were killed and injured, and we stand by the people of Lebanon and renew our commitment to a stable, sovereign, and independent Lebanon."

National Security Council spokesman Tommy Vietor said in a statement there is "no justification for using assassination as a political tool." He says the U.S. will stand with the Lebanese government to bring to justice those responsible "for this barbaric attack."

Sunni-Shiite tensions
Tension between Sunnis and Shiites has been rumbling in Lebanon ever since the end of the 1975-1990 civil war but reignited after the Syria conflict erupted.

It reached its peak when Hariri, a Sunni, was killed in 2005. Hariri supporters accused Syria and then Hezbollah of killing him -- a charge they both deny. An international tribunal accused several Hezbollah members of involvement in the murder.

Clashes over Syrian conflict in Lebanon leave ten dead

Hezbollah's political opponents, who have for months accused it of aiding Assad's forces -- have warned that its involvement in Syria could ignite sectarian tension of the civil war. 

However fighting had broken out this year between supporters and opponents of Assad in the northern city of Tripoli.

Reuters, The Associated Press and NBC News' Paul Nassar contributed to this report.

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TRIPOLI, Lebanon – It only takes a two-minute stroll down Syria Street to see why so many people are so worried about what might happen next in Lebanon.

A hole punched through the wall of the mosque by a rocket or mortar shell, smoke-blackened masonry, shops and apartments bearing the pockmarks of fierce gun battles.

Syria Street is the aptly named thoroughfare that separates rival factions in Lebanon’s second city.

For much of the past week, the two sides have been waging a mini-civil war.

It is a direct spill over from the chaos in neighboring Syria.

Photos: Violence on the streets of Tripoli

One side of the street is home to a hard-line Sunni Muslim militia who run guns to rebels across the border.

“President Assad is trying to destroy us,” says Sheik Bilal Masri, by way of explanation. “They cause trouble here to take the pressure of them in Damascus.”

We meet a small group of his men. They are well-armed and apparently spoiling for a fight.

Not many yards away, posters of Syria’s President Bashar Assad striking stern military poses adorn walls on the other side of the street.

Here the people share Assad’s Alawite faith and, it seems, the same determination to defend his regime.

Omar Ibrahim / Reuters

A man hides behind sandbags amid clashes in the Bab al-Tebbaneh neighborhood in Tripoli, Lebanon, on Thursday.

“No one wants a civil war in Lebanon,” a local Alawite leader tells me.  “But everyone should be warned: There will be repercussion for anyone who tries to meddle in Syria.”

Conflict along Syria Street is nothing new. But the outside world began to take notice on Monday when for the first time in four years, gun battles broke out on the streets of Lebanon’s capital, Beirut.

2 killed, 18 hurt as Syria conflict spills over into Lebanon

It was a brief glimpse back into the abyss for a nation scarred by years of civil strife.

In 2005, Syrian troops were forced to withdrawal from Lebanon, but Damascus is still a big player in the fractured politics of a country that sees rival Muslim and Christian sects share power in a set of uneasy alliances.

Syria’s most powerful friend here is Hezbollah, the militant Shiite group that probably holds the key to whether Lebanon survives in one piece.

Inside Syria rebel stronghold: 'The city is on mute' 

Its heartland in the south of Beirut has been tense, but so far its leader, Hassan Nasrallah, has kept his forces out of the fray.

But for how long?

The fatal shooting of two Sunni clerics followed by the kidnapping of Lebanese Shiite pilgrims in Syria shows how unpredictable events have become.

A message to Assad? War games held near border

For more than two decades, Timur Goksel has watched events in Lebanon. Once of the U.N. Mission here, he now lectures at the American University in Beirut.

He tells me the country has rarely felt so dangerous.

“I hope I am wrong because this is scary. If the faction leaders lose control of these young guys with the guns then we’re in trouble,” he said.

Their bloody history has taught the Lebanese to be a fatalistic people.

“The country is at boiling point,” another seasoned observer told me with a shrug.  “What is coming will be very bad.”

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